This invention is directed to a visual chart and a method of using that chart to evaluate the visual and/or imaging system undergoing analysis in terms of both contrast sensitivity and spatial response. Specifically, this invention is an improvement on my U.S. Pat. No. 4,365,873 entitled Spatial Frequency and Contrast Sensitivity Test Chart issued Dec. 28, 1982.
There have been recently developed new protocols of vision test charts which test for the contrast perception of the subject or system being interrogated for visual contrast sensitivity and contrast acuity. An example of such a system is that disclosed in my U.S. Pat. No. 4,365,873 entitled Spatial Frequency and Contrast Sensitivity Test Chart issued Dec. 28, 1982. In the preferred embodiment in that patent, rows and columns of patches were presented against a white background. The gratings in successive rows of patches increase spatial frequency by octave steps. The patches in successive columns decrease contrast in half contrast increments. There results a system which enables measurement of contrast sensitivity and spatial frequency response.
This system has been the subject of criticism. It has been found by others that an aliasing artifact is present that appears to produce erroneous contrast sensitivity under conditions of high dioptic blur. It was suggested that the increased contrast sensitivity found at high spatial frequencies using +3 diopters of blur was due to the sharp edges and high luminance difference between the grating patches and the surround.
Aliasing, in this case, is said to be due to the interaction of the gratings with its own edges. The spatial frequency and contrast sensitivity test chart disclosed in my prior U.S. Pat. No. 4,365,873 uses small circular grating patches (about 1.4.degree. of solid angle) with an average luminance considerably lower, about 20% of its surround. It has been suggested that because the high luminance grating stripes still have lower luminance than the surround, that the contrast of the grating surround is higher than that of the grating stripes. The resulting small circular grating can have beats that are more visible than the grating stripes. These beats have the same orientation as the true gratings. They can be used by the observer to produce a correct response even though the grating stripes may not be visible to that particular observer. It has been concluded that my spatial frequency and contrast sensitivity test chart ". . . seems unsuited to understanding the effects of blur on a patient's vision . . . "
It is important to point out that the preceding criticisms will also apply to any other test charts comprised of targets having background mean luminance difference from the average of the test targets, especially those targets having sharp edges.